Certainly poor performer is touring a county town (6)
I believe the answer is:
oakham
'county town' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
'certainly poor performer is touring a' is the wordplay.
'certainly' becomes 'ok' (both are affirmative answers).
'poor performer' becomes 'ham' (I've seen this in another clue).
'is touring' indicates putting letters inside (touring can mean going round).
'ok'+'ham'='okham'
'okham' enclosing 'a' is 'OAKHAM'.
Can you help me to learn more?
(Another definition for oakham that I've seen is " Former county town in Rutland".)